1. Field of the Invention
This invention relates an encoding apparatus using orthogonal transformation.
2. Description of the Prior Art
Generally, when video signals are to be recorded or transmitted, a method of reducing information amount by the high efficiency encoding technology has been used as a powerful means because the information amount is extremely large. Here, as an example of the compression high efficiency encoding technology, there has been known orthogonal transformation such as, for example, the Hadamard transformation and discrete cosine transformation (DCT). With the orthogonal transformation encoding technology, inputted video signals are blocked, frequency-divided in block unit to obtain frequency components and the frequency components thus obtained are respectively encoded, thus reducing the information amount to be allocated to the high frequency component with less influence on visual deterioration. As a result, the information amount of the original video signal can be reduced.
However, a conventional encoding apparatus has the following problems;
The accuracy of orthogonal transformation depends on the output word length after orthogonal transformation. On the one hand, when the output word length is large, the accuracy of transformation is improved, and though deterioration due to transformation or inverse transformation becomes small, the number of quantizers and the scale of an encoding table to be used become large for the reason that dynamic range becomes large, resulting in the increase of the circuit scale for encoding. On the other hand, when the output word length of orthogonal transformation is small, for the reason that the dynamic range becomes small, the circuit scale for encoding can be made small, but an enough accuracy cannot be obtained in performing transformation and inverse transformation.
In addition, referring to the quantizers to be used, the number of them becomes large, and the accuracy of encoding can be improved by selecting the quantization width finely. However, when m quantizers are used, it is required to calculate the data amount when m kinds of quantization are carried out for each block, which means that the circuit scale for encoding becomes large with an increase in the kind of quantizer to be used.